Sometimes, explaining a part of the dredging process can be as simple as seeing through the mixture. In this exhibit we can demonstrate what happens beneath the surface of the cargo in the hopper1,2,3. Along the way, we explain some quirky behaviour in other phases of the dredging process, also. The exhibit consists of five tubes in a frame, that can rotate around a horizontal axis. In the tubes are various types of soils. Each with their own settling behaviour. The exhibit was recently added to the Damen Dredging Experience. One more reason to highlight it here.
One major part in the dredging process is the hydraulic transport of particles in a carrier fluid. Pickup and transport have been touched upon in previous posts4,5. Here we concentrate on the end of the process: settling and deposition. This can be either in a hopper or on the discharge area. In both cases you will only see the fluid surface during the process and at best the top of the deposited sediment. How the material came there, was deposited and stacked up can’t be readily seen. As the tubes allow these processes to be observed from the side, we can follow the events.
The exhibit can be started by upending the frame with the tubes. The material that sat in the lower end gets now on the top end. They all are released at the same time and we see immediately see the differences in settling velocity for the different particle sizes6. The gravel falls down within ten seconds. The sand is slower and the clay even has problems getting started. One nice observation is the mixture of soils. Against the height of the tube, the fractions in the sample are released simultaneously. Still, the fractions separate over the fall height and stack up again in their original order. This not only happens in the tube. In the hopper or the discharge area, a widely graded sediment will sort itself to the various fractions.
Although for all the samples the particles are released simultaneously, you can still see a slight difference in settling velocity within each sample. This can be either due to slight variations in size that are possible within each mesh size used for sieving. Another cause for the differences might be the differences in shape. A perfectly spherical particle will have a faster settling velocity than an oddly shaped potato7.
And even then, the initial particles that fall down have a greater velocity than the particles in the bulk of the sample, even when having the same particle size and shape. This is due to the water flowing up around the particle. The upward flow is slowing down an adjacent particle. This interaction is called ‘hindered settling’. At high concentrations this can contribute to the efficiency of pipe line transport8. But for the settling it is really hindering the loading time.
At the very end of the settling, the particle gets deposited at the bottom, or on top of another. The water that is caught in between has to escape. This causes one last puff of fluid to flow upward and take the very find dust present upward. This happens with each particle that settles and causes the layer of dust to lift to the surface of the deposited sediment. So even when loading a cargo of gravel, you will always end up with a layer of dust on top. So, don’t judge the quality of the cargo just by the dust you see on top. Take a deeper sample or base your evaluation on the signals from the sensors from the screening tower.
- Hopper Loading: What Happens Beneath the Surface, Discover Dredging
- Graduation of Ben Sloof: Hopper Loading Model and Overflow Losses, Discover Dredging
- IADC Young Author Award for 1DH Hopper Loading Model of Jordy Boone, Discover Dredging
- Loose Sand, How Hard Can it Be? Discover Dredging
- Graduation of Arend van Roon: Detecting Flow Regime And Optimising Transport Efficiency, Discover Dredging
- Terminal velocity, Wikipedia
- Sphericity, Wikipedia
- Slurry Transport Fundamentals, Limit Deposit Velocity Framework – 2nd Edition, SA Miedema